Fix the explanation of HTTPNotFound for cell showing v2 API

This patch adds the explanation to cell showing in v2 API, when
the cell is not found, the exception of HTTPNotFound will return
more infomation.

Change-Id: I25dee08a04ad974af5309999527e0f173d2c2cab
Closes-Bug: #1316152
Downgrade and fix log message for floating ip already disassociated

When a floating IP is associated, try to disassociate may
get an FloatingIpNotAssociated exception. This patch
fixes the log message and downgrades the log
level as it is something that will reasonably occur.

Change-Id: I1d4d5fe916d99024ff4df53d300a0f26d74eb102
Fix the duplicated extension summaries

If operating `nova --os-compute-api-version 3 extension-list`, the
summaries of both flavor-extra-specs and flavors extensions are the
same "Flavors Extension.". The summary of flavor-extra-specs should
include its feature.
There is a same issue on os-extended-server-attributes and
os-extended-availability-zone extensions also.

This patch fixes these summaries.

Change-Id: I6778229221ca80632b3a303745ff5133488f327d
Related-Bug: #1318973
Merge "Fix the duplicated extension summaries"
Merge "Fix the explanation of HTTPNotFound for cell showing v2 API"
Merge "Downgrade and fix log message for floating ip already disassociated"